Borderline personality disorder (BPD) is extremely debilitating, with 75% of diagnosed individuals engaging in suicidal behavior and 5-10% completing suicide. Once the disorder emerges, it is difficult to treat and places an enormous burden on mental health systems. Thus, longitudinal research that begins in early adolescence is urgently needed to identify precursors of BPD features that can inform prevention and early intervention programs. Developmental theory postulates that emotional instability (EI) in the context of invalidating parenting behaviors underlies the development of BPD features, including suicide and other impulsive behaviors, abandonment fears, relationship volatility, and difficulties with identity formation. EI is specified as dimensions of variability in negative affective states, intensity of affect, and slow recovery following the onset of negative affect. However, the biological underpinnings of EI components and their predictive utility for BPD features are not known. Understanding how parenting behaviors and youth EI dampen or exacerbate each other over time as well as influence the development of BPD features is essential for the identification of clinically modifiable parent-youth transactions. Aims: The current application tests hypotheses about components of EI as precursors of BPD features and the developmental course of these features. Specifically, we will examine concurrent, prospective, unique and shared associations between components of EI, BPD features, and internalizing and externalizing problems within the parenting context. Thus, the proposal also examines reciprocal associations between parental response to emotion and youth EI and the potential moderating role of parental response to emotion on the development of BPD features. Approach: We will use a multi-modal, ecologically valid assessment battery of EI in young adolescents, emotion-focused parenting strategies, and parent-youth interactions. We will examine these processes in real-time (using an ecological momentary assessment protocol, observational ratings, and autonomic reactivity monitoring) as well as chart the course of their development with three assessments over 18 months. We will determine how specific EI components are related to near-neighbor internalizing and externalizing problems (e.g., depression, conduct problems), but we propose that the combination of these three EI components form a specific marker of subsequent BPD features, especially in the context of deleterious parenting strategies. Youth aged 11-13 years and primary caretakers (N=165 families; H50% girls) will be recruited from psychiatric clinics, and adolescents will be screened to ensure that we oversample severe and clinically significant levels of EI. In accordance with NIMH strategic priorities, the proposed work is a novel and critical step to identify who is at greatest risk and how to intervene with vulnerable youth and their families.